Russian investigators today said that terrorism was the most likely cause of last night's devastating train crash in which at least 26 people were killed and nearly 100 injured when their luxury express from Moscow to St Petersburg derailed.

The head of Russia's railways, Vladimir Yakunin, said he believed an "unidentified device" had exploded under the train, sending three carriages packed with passengers hurtling off the rails.

"As far as theories go… our main version is that this was an explosion of an unknown device, by unknown individuals. Put simply, it was an act of terror," Yakunin said today at the crash scene.

The Nevsky Express, carrying 661 passengers, derailed at 9.34pm yesterday, close to the village of Uglovka, 200 miles (320km) north of Moscow. This afternoon Russia's health minister, Tatyana Golikova, warned the death toll could rise, with 18 people still unaccounted for.

So far investigators have not made clear who they believe might have been responsible. But Yakunin said the incident was "analogous" to another derailment on the same line three years ago, also involving the Nevsky Express, in which 19 people were injured.

Russian prosecutors blamed the earlier derailment on Chechen extremists, who have been fighting an on-off war against the Russian state for two decades. There are daily attacks on security forces in the republics of Ingushetia, Chechnya and Dagestan.

Witnesses described how they heard a "tremendous crash" as the train derailed. "At exactly 9.30pm, 15 minutes after we had passed Bologoye, we heard an almighty crash," survivor Boris Gruzd told the radio station Ekho Moskvy. "It seemed to me as if we had lost a wheel or smashed through some kind of obstacle. I didn't hear any explosion," he said.

Gruzd said the train driver braked hard. The passengers then spent 30 minutes unaware that the last three carriages of the 14-carriage train had been thrown from the tracks.

"The first wagon numerically was 1.5-2km away from the rest of the train. The second had completely flipped over. The third had come off the rails, but was near the main part of the train and was still standing vertically. As far as I know nobody from this wagon was seriously hurt."

Gruzd said it was extremely difficult to reach passengers trapped in the mangled carriages, with rescuers peering into the gloom and using flashlights.

President Dmitry Medvedev called for calm over the incident. "We need there to be no chaos, because the situation is tense as it is," he told Russian TV.

Government officials last night said investigators had found a 1 metre crater under the rails. Reuters reported today that its reporters at the scene had been unable to see it. Earlier, Russian news agencies had quoted transport officials as saying the cause may have been an electrical fault. Russia has a record of serious accidents caused by Soviet-era infrastructure.

According to the radio station Echo of Moscow, a far-right group opposed to migrants from the former Soviet republics of central Asia has claimed responsibility for the crash, which has delayed 27,000 passengers.

The Kremlin is likely to blame Islamist extremists waging a guerrilla campaign in the north Caucasus. Rebel fighters have carried out numerous attacks in recent months, including suicide bombings, in an apparent attempt to establish an Islamic caliphate in the region.

Russian prosecutors believe Pavel Kosolapov, an ex-solider and former associate of the late Chechen rebel leader Shamil Basayev, masterminded the previous derailment. He is on the run but two residents of Ingushetia have been charged with helping to carry out the attack.